Heavier summer downpours with climate change
2 Jun 2014 02:52 PM
Extreme summer rainfall
may become more frequent in the UK due to climate change, according to new
research led by the Met Office in collaboration with Newcastle
University.
The new study, from the joint
Met Office and NERC funded CONVEX project, uses a state-of-the-art climate
model providing the first evidence that hourly summer rainfall rates could
increase.
While summers are expected to
become drier overall by 2100, intense rainfall indicative of serious flash
flooding could become several times more frequent.
The results from the study,
published in Nature Climate Change, are the first
step towards building a more complete picture of how UK rainfall may change as
our climate warms.
Dr Lizzie Kendon, lead author of
the research at the Met Office, said: "Until now, climate models
haven't been able to simulate how extreme hourly rainfall might change in
future. The very high resolution model used in this study allows us to examine
these changes for the first time.
"It shows heavier summer
downpours in the future, with almost five times more events exceeding 28mm in
one hour in the future than in the current climate - changes we might expect
theoretically as the world warms. However, we need to be careful as the result
is only based on one model - so we need to wait for other centres to run
similarly detailed simulations to see whether their results support these
findings."
As the atmosphere warms it can
hold more moisture and this is expected to intensify rainfall. However,
research is needed to understand what this might mean for extremes and how this
might affect the UK.
In winter it is the daily or
multi-day rainfall totals that are important, because we tend to get steady,
long-lasting periods of rain from large scale weather systems - similar to
those seen during the winter floods of 2013/14.
Climate models, which generally
work at coarse resolutions, have been able to accurately simulate winter
rainfall and have suggested generally wetter winters with the potential for
higher daily rainfall rates in the future.
In summer, however, it is the
hourly rates that are more important as rain tends to fall in short but intense
bursts - as seen during the Boscastle flooding of 2004 and 'Toon Flood'
in Newcastle in 2012. Climate models have so far lacked the resolution to
accurately simulate the smaller-scale convective storms which cause this type
of rain.
To deal with this issue, this
study uses a climate model with a higher resolution than ever used before to
examine future rainfall change - using 1.5km grid boxes instead of the usual
12km or larger - the same as the Met Office weather forecast model. This model
gives a realistic representation of hourly rainfall, allowing us to make future
projections with some confidence.
It was so computer intensive
that only the southern half of the UK could be studied and even then it took
the Met Office supercomputer - one of the most powerful in the world - about
nine months to run the simulations.
These simulations looked at two
13-year periods, one based on current climate and one based on expected climate
around 2100.
Prof Hayley Fowler, from Newcastle University's School of
Civil Engineering and Geosciences, and lead on the CONVEX project, added:
"We need to understand about possible changes to summer and winter
rainfall so we can make informed decisions about how to manage these very
different flooding risks in the future. The changes we have found are
consistent with increases we would expect in extreme rainfall with increasing
temperatures and will mean more flash floods.
"The next steps are to see
if these changes are consistent with observed trends in summer rainfall
extremes and changes projected by climate models in other parts of the world.
We will be looking at this over the next five years, jointly with the Met
Office and other leading international scientists in the European Research Council funded
INTENSE project."